Explicitly record Tombstone quotes as deleted

This adds a `deleted` state to the internal representation, but this does
not change the API, which already included such a state.

app/lib/activitypub/parser/status_parser.rb

@@ -121,6 +121,10 @@ class ActivityPub::Parser::StatusParser
     %w(quote _misskey_quote quoteUrl quoteUri).any? { |key| @object[key].present? }
   end
 
+  def deleted_quote?
+    @object['quote'].is_a?(Hash) && @object['quote']['type'] == 'Tombstone'
+  end
+
   def quote_uri
     %w(quote _misskey_quote quoteUrl quoteUri).filter_map do |key|
       value_or_id(as_array(@object[key]).first)